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Leedsichthys was around the size of a blue whale, making it the largest fish of all time. Despite its daunting size, Leedsichthys was a gentle giant with a simple lifestyle. It swam close to the ocean's surface, taking in mouthfuls of water filled with plankton, tiny shrimps, jellyfish and small fish, which it sieved through the mesh plates at the back of its mouth. The plates were made from tens of thousands of fine teeth.

Leedsichthys traveled to parts of the world where nutrient-rich, deep-sea currents would rise to the ocean surface. The excess of nutrients in these currents would feed the plankton, which would become so plentiful that it formed a dense, organic soup. Such a large amount of plankton would attract shoals of Leedsichthys intent of gorging themselves. Such feasts could last for weeks, after which a large animal like Leedsichthys would be able to survive for several more weeks without feeding. During this time, it would shed the filter plates at the back of its mouth and would wait while new ones grew back.

Leedsichthys, despite their size, were not completely safe from predators. It had no means of defending itself against large and persistent attackers. Leedsichthys could be attacked by animals like Metriorhynchus, Hybodus and particularly Liopleurodon.